Wikipedia is an online encyclopaedia, arguably the most popular of its genre today.
If you might have noticed, the first page of any search term on google throws up wikipedia, thus showing that it is an authority site.It is also a high PR site, if you check its PR using any page rank tool.
Because of the fact that it allows users to edit articles, many webmasters have been guilty of spamming wikipedia with their sites which aren't really useful to the community.Its editors remove the links, but it often turns to be a time consuming exercise.
Enforcing the no follow link will surely reducing in the amount of spamming that this valuable resource is currently being subjected to.
Kudos to them.